linux-kernel - Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

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Date: Mon, 10 Jun 2024 06:55:56 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>
Cc: linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
 cho@...rosoft.com, decui@...rosoft.com, John.Starks@...rosoft.com
Subject: Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

On 5/28/24 03:09, Kirill A. Shutemov wrote:
> Currently, attempting to perform MMIO from userspace in a TDX guest
> leads to a warning about an unexpected #VE and SIGSEGV being delivered
> to the process.

Does it _always_ result in a #VE?  Or is this only when guests mmap()
something like from a driver and the host doesn't back the shared memory?

> Enlightened userspace may choose to handle MMIO on their own if the
> kernel does not emulate it.
> 
> Handle the EPT_VIOLATION exit reason for userspace and deliver SIGBUS
> instead of SIGSEGV. SIGBUS is more appropriate for the MMIO situation.

Is any userspace _actually_ doing this?  Sure, SIGBUS is more
appropriate but in practice unprepared userspace crashes either way.

> @@ -641,17 +647,20 @@ static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
>  	switch (ve->exit_reason) {
>  	case EXIT_REASON_CPUID:
>  		return handle_cpuid(regs, ve);
> +	case EXIT_REASON_EPT_VIOLATION:
> +		if (is_private_gpa(ve->gpa))
> +			panic("Unexpected EPT-violation on private memory.");
> +
> +		force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)ve->gla);
> +
> +		/* Return 0 to avoid incrementing RIP */
> +		return 0;

This _really_ needs a comment, probably even a helper function where you
can actually explain what is going on.

I could barely remember what this is for today.  There's no hope for me
in a couple of years.

Just thinking through the possibilities here:

Private=> Private      	: no #VE
Private=> Anything else	: fatal shutdown
Shared => Shared	: no #VE
Shared => Private	: #VE (end up here)
Shared => !Present      : #VE (end up here)

So I think you're trying to differentiate between the last 2 cases.
"Shared => !Present" is the normal case where today the VM wants to
generate a VMEXIT.  We'll probably get these from setups where somebody
is trying to do good ol' device emulation but in TDX.

"Shared => Private" is an actual kernel bug.  Why panic() though?  Do we
*know* the system is unstable at this point?  Why not just dump an
error, send a fatal signal, and move on?